Activation of cytotoxic T lymphocytes is routinely studied by following the fate of antigen-bearing target cells in, for example, the .sup.51 Cr-release assay. Although this assay is informative and easy to perform, it is not a direct assay for cytotoxic T lymphocyte activation, since the presence of another cell, a target cell, is required to provide a "read-out" system, thereby superimposing activation events in cytotoxic T lymphocytes with destructive "lethal hit" processes in the target cells.
The presence of target cells in the cell mixture greatly complicates the study of biochemical events in cytotoxic T lymphocytes. For example, the use of target cells as an activating stimulus does not allow control of the proportion of activated cytotoxic T lymphocytes, because of the relatively low efficiency of engagement of cytotoxic T lymphocytes in the cytotoxic T lymphocyte-target cell conjugates, as well as the difficulties in manipulating the density of activating ligand on the surface of the target cells.